1. Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, to an in-plane switching mode liquid crystal display device.
2. Description of the Background Art
Recently, demand for light, thin, short, and small flat panel display devices has been increasing for use in various mobile electronic devices, such as mobile phones, PDAs, or notebook computers. Thus, research is active in areas related to flat panel display devices, such as Liquid Crystal Display (LCD), Plasma Display Panel (PDP), Field Emission Display (FED), Vacuum Fluorescent Display (VFD). Among these display devices, the LCD has taken the spotlight because it can be readily mass-produced, requires simple driving circuits, and provides high picture quality. An LCD device displays an image by controlling light using an electric field. The LCD device includes a liquid crystal panel on which pixels are arranged in a matrix, and a driving circuit for driving the liquid crystal panel.
In general, pixels on the liquid crystal panel are driven by an inversion driving method such as a frame inversion system, a line inversion system and a dot inversion system. Amongst the inversion driving methods, the frame inversion system inverts the polarity of a data signal supplied to pixels on the liquid crystal panel whenever a frame is changed. The line inversion system inverts the polarity of data signals supplied to corresponding pixels according to each gate line. The dot inversion system supplies a data signal with an opposite polarity to horizontally and vertically adjacent pixels adjacent, and inverts the polarity of data signals supplied to each pixel on the liquid crystal panel at each frame. Among the inversion driving systems, the dot inversion system provides an image with excellent picture quality compared with the frame and line inversion systems.
FIG. 1 is a plane view of an in-plane switching (IPS) mode LCD device according to the related art. As shown in FIG. 1, a gate line 1 and a data line 10 are arranged on a first substrate of a liquid crystal panel. The gate line 1 and the data line 10 cross each other to define a pixel region on the first substrate. A gate electrode 9, a semiconductor layer (not shown), and source/drain electrodes 13 and 15 are formed at the crossing of the gate line 1 and the data line 10, forming a switching device, for example, a thin film transistor (TFT).
A pixel electrode 3 and a common electrode 5 are alternately disposed at each pixel region of the liquid crystal panel to generate an in-plane electric field on the first substrate. The pixel electrode 3 receives a data signal from the source/drain electrodes 13 and 15 of the switching device (TFT). The in-plane electric field is generated between the pixel electrode 3 and the common electrode 5. The gate electrode 9 and the source/drain electrodes 13 and 15 are electrically connected with the gate line 1 and the data line 3, respectively, to turn on the switching device (TFT) with a signal inputted through the gate line 1 and transfer a data signal applied through the data line 3 to the pixel electrode 3. As a result, the LCD displays an image by controlling the light transmittance of a liquid crystal layer with the electric field formed between the pixel electrode 3 and the common electrode 5 according to the data signal supplied to each pixel region. A color filter layer (not shown) is formed on a second substrate, and a liquid crystal layer is formed at a separated space between the first and second substrates.
In such an IPS mode LCD, because liquid crystal molecules of the liquid crystal layer are driven by the in-plane electric field, the viewing angle is increased compared to the conventional TN (Twisted Nematic) mode LCD. Specifically, a viewing angle of about 80°-85° in all directions (up/down and left/right direction) may be obtained.
The related art LCD device is generally driven by a frame frequency of 60 Hz. In a system that requires low power consumption, such as a notebook computer, the frame frequency may be lowered to about 50-30 Hz. An LCD panel driving method has been proposed which uses a horizontal 2-dot inversion system and requires low power consumption.
However, in the conventional IPS mode LCD, because the data line 10 and the pixel electrode 3 are adjacently disposed in parallel, a signal interference is easily generated between the data line 10 and the pixel electrode 3, causing crosstalk and light leakage.
To solve the crosstalk and light leakage problems, an outermost common electrode 5′ is disposed near the data line 10. The outermost common electrode 5′ is larger than other common electrode 5. However, such an electrode disposition structure degrades the aperture ratio of the LCD and distortion of the electric field due to the signal interference from the data line cannot be effectively prevented.
FIGS. 2A and 2B are enlarged views of a portion ‘I’ of FIG. 1 illustrating a distortion of the related art liquid crystal array due to a signal interference according to a voltage variation on the data line. Referring to FIGS. 2A and 2B, a rubbing direction for inducing an initial arrangement of liquid crystal molecules has a tilt of about 45° with respect to the common electrodes 5 and 5′ and the pixel electrode 3. An in-plane electric field generated when a voltage is applied to the common electrodes 5 and 5′ and the pixel electrode 3 is perpendicular to the common electrodes 5 and 5′ and the pixel electrode 3.
As shown in FIG. 2A, when a voltage of 8V is applied to the data line 10 and voltages of 5V and 8V are respectively applied to the common electrodes 5 and 5′ and the pixel electrode 3, a direction of liquid crystal molecules is determined in a first direction 30 by the electric field generated due to a voltage difference between the common electrodes 5 and 5′ and the pixel electrode 3. As shown in FIG. 2B, if the voltage applied to the data line 10 changes from 8V to 10V, the electric field generated on the actual driving region of the liquid crystal molecules rotates from the first direction 30 to a second direction 35.
The voltage change on the data line distorts the direction of the electric field in the pixel region, causing a change in the arrangement of liquid crystals. As a result, although the same voltage is applied to the common electrodes and the pixel electrode, the perceived color changes on the display.